How to link user databases for 2 wordpress sites

Hi I am running my main wordpress website and I have decided to expand on it by adding a sub domain with another installation with the job roller theme.
Specs:
MainEuropeband.eu
sub domainmusicians.europeband.eu
Both installations are running WP versions 3.4.2
I am also running buddypress version 1.6.1 on my main europeband.eu installation.

Basically what i want to achieve is for users to have the ability to register/login in into either the main and sub domain websites so they dont have to register or log in twice.

I have found a few ‘tutorials’ or support convos on the wordpress forum however they are a little dated and I thought it would be safe to raise the topic again.

An explanation on how to execute this task would be great and much appreciated.

Thanks
John